1. Field of the Invention
This invention relates to electrical circuit means for measuring the mass flow of an incident fluid using a resonantly vibrating object.
2. Prior Art
A key aspect of maintaining the correct ratio of air to fuel at the intake of fuel-injected internal combustion engines is measuring the mass flow (kg/sec.) of incoming air. From this, the amount of incoming oxygen can be inferred so that the correct amount of fuel can be injected. In one method, the well known hot-wire anemometer, utilizing the phenomenon of forced convection has seen considerable development. An alternate measurement technique, U.S. Pat. No. 4,024,759 issued to Klinger et al, teaches how the aerodynamic drag force, resulting from the incident flow of a gas on a resonantly vibrating object can be used to measure the mass flow of the incident gas. This method, which is extendable to fluids in general and may be classified in the category of drag as opposed to thermal anemometry, has several advantages over using the hot-wire anemometer both with regard to function and economy of manufacture. For example, the drag anemometer could be used to measure the mass flow of combustible-gas mixtures for which application of a hot-wire device might be unsafe.
With the vibrating object (e.g. in the form of a flat blade) in the flow, mass flow is determined by the amount by which the drag force damps the vibration. To measure mass flow, an actuator will be required to excite vibrations as well as a sensor to measure the amplitude of the vibration. Since the peak amplitude of a resonance vibration is inversely proportional to the damping, measurement of that amplitude will provide a measure of the mass flow. The mechanical vibrators may have a sharp resonance in the frequency domain. Said differently, they may be high-Q devices. If one measures peak amplitude to determine mass flow, the response time, T, for a step change in mass flow to change the amplitude is proportional to Q. Thus a high-Q device (which as discussed below offers desirable benefits) is realized at the expense of response time. To overcome this difficulty, a negative feedback control circuit is used to maintain ac vibrational amplitude constant despite damping. The control voltage used to accomplish this increases with flow-induced damping and measures mass flow. Because the vibrational amplitude does not change, response time is improved. U.S. Pat. No. 4,024,759 teaches such a circuit but employs complex digital circuitry.
U.S. Pat. No. 4,488,439 issued to Gast et al teaches improved analog circuitry, but employs band-pass filters which establish the desired response time of the device. Applicant's invention teaches an improved circuitry which simply maintains constant vibrational amplitude without band pass filters and thus realizes short response times.
In addition to flow-induced damping, all vibrators have inherent damping in zero flow due to frictional effects. Such effects would include viscous losses, for example, in epoxy layers or piezoelectric elements used to actuate or sense the vibrations. It is desirable for these inherent frictional effects to be low (resulting in a high Q vibrator) since they occur simultaneously with the flow-induced damping and reduce sensitivity to that damping component especially at low flows. The patent to Gast et al teaches circuitry to effectively eliminate the inherent damping by electronically subtracting a signal proportional to the inherent damping from that voltage which measures total damping. The method employed involves the use of an additional electrical oscillator as well as band-pass filters which will increase time response. It would be desirable to accomplish this function with a simpler (no additional oscillator is required) and faster (no band-pass filter is required) supplement to the feedback circuit. It would also be desirable to allow for a calibration of the response of the device to overcome component and manufacturing variations. These are some of the problems this invention overcomes.
Further, due to component and manufacturing variations, the exact resonant frequency of the mechanical vibrator may vary somewhat with each device. Circuits of the prior art allow one to maintain oscillation at the resonant frequency despite this variability. However, the resonant frequency of the device may change during use. For example, dust or ice may accumulate on the vibrating element thereby changing its effective mass and resonant frequency. Although existing circuitry would track this change (to the extent allowed by band-pass filters) such in-use change is undesirable. The reason is that the amplitude of peak vibration (which is held constant) is inversely proportional to the product of the total damping coefficient (C) and the angular resonance frequency (.omega..sub.o). After an initial calibration corresponding to a particular initial .omega..sub.o, any further change in W.sub.o during use cannot be distinguished from a change in damping resulting from flow. Thus, it is desirable to keep .omega..sub.o constant after an initial calibration. It would be desirable to allow the resonance frequency to be "pulled" and hence maintained at a constant value during operation thereby preserving a fixed calibration. Within the frequency "pulling" circuit is a voltage indicating the extent of the frequency modification. This information may be useful in some applications. If it is not, a simplification is possible in which the velocity of the blade is held constant rather than the amplitude. In this method the mass flow can be measured independently of small changes in the resonant frequency which may occur due to manufacturing variations or in process use. This is another problem this invention overcomes.